Metal castings, having either an equiaxed, columnar grain, or single crystal microstructure, are widely used in the turbine section of modem gas turbine engines. Frequently, these castings are used as turbine blades, and they are subjected to some of the most severe operating conditions of all parts used in the engine. Because of the demands placed upon these parts, and the critical nature they play in the overall performance of the engine, the parts are fabricated from alloys called superalloys, which have an optimum balance of mechanical strength and resistance to oxidation and hot corrosion. The mechanical strength characteristics which are required of turbine section components include creep strength and resistance to thermal fatigue.
Turbine blades have an airfoil portion and a root portion; typically, the root portion has a fir-tree design. The blades are assembled to a turbine disk which has slots appropriately machined to allow the root portion of the blade to slide into the slot. A variety of designs are utilized to prevent the blade from sliding out of the disk slot during operation of the engine.
As indicated above, the airfoil portion of the blade is exposed to the most rigorous combination of temperature and stress conditions during engine operation; creep strength is a major design requirement for the airfoil portion of the blade. Insufficient creep strength can cause catastrophic failure during use in the engine.
While somewhat shielded from the elements during engine operation, the root portion of the blade also experiences a combination of stress and elevated temperature conditions that can cause cracking in the attachment area of the blade root. These cracks can also cause the blade to fail. The stresses that result in crack formation are primarily associated with low and high cycle fatigue. Attachment strength is a major design requirement of the root portion of the blade.
The engineering difficulties of achieving an optimum combination of high temperature creep strength and lower temperature attachment properties in a turbine blade are well known to those skilled in the art. The difficulties exist because alloy compositions and casting processes that are well adapted for producing desirable levels of creep strength for the airfoil portion of the part do not usually produce desirable attachment properties for the root portion of the part. In particular, the compositions and fine grain sizes that are required for superior attachment strength produce components that have marginal creep strength; conversely, the compositions and casting processes that are required for superior creep strength produce pans that have marginal attachment properties for advanced high stress applications.
One way that the attachment strength of cast blades made of creep resistant materials can be improved is by peening the root with either glass or steel shot. The peened blade root has better resistance to the formation of fatigue cracks than the unpeened blade root, because peening forms residual compressive stresses at the surface of the root, providing it with better resistance to crack initiation. However, as engineers attempt to design engines with increased thrust and performance capabilities, the temperatures in the turbine section become higher; if these are sufficiently high, they can accelerate the rate at which the compressive stresses (due to peening) are annealed from the blade root. Furthermore, to achieve and improve performance, engineers increase rotors speeds, which raise stress levels in the root and reduce blade root attachment life.
Another way that engineers have tried to improve the attachment strength of blades made of creep resistant materials is the bi-cast process. In the first step of this process, the airfoil portion of a turbine blade is fabricated from an alloy in such a manner to optimize creep strength. Then, molten metal of a different composition is cast around the airfoil portion in such a manner to produce a finer grained root structure having better attachment properties. See, e.g., U.S. Pat. No. 4,008,052. Bi-cast components have, unfortunately, not achieved commercial success due to the inability of the process to produce a high-integrity bond joint between the airfoil and root portions. In particular, it is very difficult to control the cleanliness of the interface between the airfoil and root portions, and to control the complicated melting and solidification processes at that interface. It is also very difficult to inspect the quality of the interface itself. Finally, the casting processes are unable to produce grain sizes in the root area that are truly free enough for optimum attachment properties; grain sizes are generally no smaller than 250-625 microns (10-25 mils).
A variation of the bi-cast process involves diffusion bonding separately fabricated airfoil and root portions to each other, as shown in U.S. Pat. No. 4,592,120. This patent describes a method for diffusion bonding an airfoil portion fabricated from a single crystal alloy having desirable creep strength, such as CMSX2, to a root portion fabricated from a powder metal disk alloy having desirable attachment strength, such as Astroloy. The two components are bonded together using a boron-enriched bonding alloy and a bonding temperature of 1,205.degree. C. (2,200.degree. F.). Like the aforementioned bi-cast process, the diffusion bonding process has not achieved widespread commercial success for many of the same reasons recited above. A further deficiency of the diffusion bonding process is that the elevated bonding temperatures can cause grain growth of the fine Astroloy grains, thereby decreasing the attachment strength of the root. The process also introduces a potentially undesirable element, in this case, boron, into the casting.
As a result of the inadequacies of these prior art processes, the gas turbine engine industry continues to search for ways to improve the fatigue strength of the turbine blade root while retaining optimum creep strength in the airfoil.